1. Tool Wear, Tool life & Machinability

2. Tool Life Useful cutting life of tool expressed in time
Time period measured from start of cut to failure of the tool
Time period b/w two consecutive resharpenings or replacements.

3. Ways of measuring tool life
No. of pieces of work machined
Total volume of material removed
Total length of cut.
Limiting value of surface finish
Increase in cutting forces
Dimensional accuracy
Overheating and fuming
Presence of chatter

4. Modes of tool failure Temperature failure Rupture of the tool point
Plastic deformation of CE due to high temp
Cracking at the CE due to thermal stresses.
Rupture of the tool point
Chipping of tool edge due to mechanical impact
Crumbling of CE due to BUE
Gradual wear at tool point
Flank wear
Crater wear

5. Tool wear
Tool wear causes the tool to lose its original shape- ineffective cutting
Tool needs to be resharpened

6. Causes of Tool Wear Attrition wear Diffusion wear Abrasive wear
Electrochemical wear
Chemical wear
Plastic deformation
Thermal cracking

7. Attrition wear At low cutting speeds
Flow of material past cutting edge is irregular and less stream lined
BUE formed and discontinuous contact with the tool
Fragments of tool are torn from the tool surface intermittently
High
Slow and interrupted cutting
Presence of vibrations
Found in carbide tools at low cutting speeds

8. Diffusion wear
Diffusion of metal & carbon atoms from the tool surface into the w/p & chip.
Due to
High temp
High pressure
Rapid flow of chip & w/p past the tool
Diffusion rate depends on the metallurgical relationship
Significant in carbide tools.

9. Abrasive wear Due to Contributes to flank wear
Presence of hard materials in w/p material.
Strain hardening induced in the chip & w/p due to plastic deformation.
Contributes to flank wear
Effect can be reduced by fine grain size of the tool & lower percentage of cobalt

10. Electrochemical wear When ions are passed b/w tool & w/p
Oxidation of the tool surface
Break down of tool chip tool interface

11. Chemical wear Interaction b/w tool and work material
Plastics with carbide tools
Cutting fluid

12. Plastic Deformation
When high compressive stresses acts on tool rake face- tool deformed downways – reduces relief angle
Modifies tool geometry and accelerates other wear processes

13. Thermal cracking Due to cyclic thermal stresses at cutting edge
Comb cracks
Transverse cracks
Chipping of tool

14. Geometry of tool wear
Flank wear (edge wear)
Crater wear (face wear)

15. Flank Wear
Tool slides over the surface of the work piece and friction is developed
Due to Friction and abrasion.
Adhesion b/w work piece & tool- BUE
Starts at CE and starts widening along the clearance face
Independent of cutting conditions and tool / work piece materials
Brittle and discontinuous chip
Increases as speed is increased.

16. Primary stage rapid wear due to very high stress at tool point
Wear rate is more or less linear in the secondary stage
Tertiary stage wear rate increases rapidly resulting in catastrophic failure.

17. Crater wear Direct contact of tool and w/p Forms cavity
Ductile materials – continuous chips
Initiates rapid rupture near to nose
Leads to
weakening of the tool
Increase in cutting temp
Cutting forces & friction

18. Measurement of tool life
Time for Total destruction in case of HSS or time to produce mm wear for carbide tools
Tool life expressed by Taylor’s eqn
VTb = C
V = cutting speed in cm/min
T= tool life in min
b= const= 0.1 for HSS
C= 50 for HSS
Cemeted carbide : b=0.125, C=100
Tool life expressed in volume of metal removed
L = TVfd

19. Measurement of tool life
Diamond indentor technique
Radioactive techniques
Test at elevated cutting speeds
Facing tests
Test with low wear criterion

20. Factors affecting tool life
Cutting speed
Physical properties of w/p
Area of cut
Ratio of feed to depth of cut
Shape and angles of tool
Tool material and its heat treatment
Nature and quantity of coolants
Rigidity of tool and w\p

21. Machinability Machinability is defined in terms of:
Surface finish and surface integrity
Tool life
Force and power required
The level of difficulty in chip control
Good machinability indicates good surface finish and surface integrity, a long tool life, and low force and power requirements
Machinability ratings (indexes) are available for each type of material and its condition

22. Factors affecting machinability of metals
Material of w/p- hardness, tensile properties, strain hardenability
Tool material.
Size and shape of the tool.
Type of machining operation.
Size, shape and velocity of cut.
Type and quality of machine used
Quality of lubricant used in machining
Friction b/w chip & tool
Shearing strength of w/p material

23. Evaluation of machinability- factors
Tool life
Form and size of chip and shear angle.
Cutting forces and power consumption
Surface finish
Cutting temperature
MRR per tool grind
Rate of cutting under standard force
Dimensional accuracy

24. Evaluation of machinability
Machinability decreases with increase in tensile strength and hardness
Machinability of a material is assessed by any of the following.
Tool life
Limiting MRR at which the material can be machined for standard short tool life.
Cutting force
Surface finish
Chip shape

25. Relative machinability
Mg alloys
Bearing bronze
Al alloys
Zn alloys
Free cutting sheet brass
Gun metal
Silicon bronze, Mn bronze
S.G Cast iron
Malleable cast iron
Gray CI
Free cutting steel
Sulphur bearing steel
Cu-Al alloys
Low carbon steels
Nickel
Low alloy steels
Wrought iron
HSS
18-8 SS
Monel
White CI
Stellite
Sintered carbides

26. Machinability index Machinability index= Vt/Vs x100
Vt – cutting speed of metal for 1 min tool life
Vs – cutting speed of standard free cutting steel for 1 min tool life.
Material MI SS 25
Low carbon steel
55-65 Cu 70
Red brass
180
Al alloys
Mg alloys

27. Machinability: Machinability of Ferrous Metals
Steels
If a carbon steel is too ductile, chip formation can produce built-up edge, leading to poor surface finish
If too hard, it can cause abrasive wear of the tool because of the presence of carbides in the steel
In leaded steels, a high percentage of lead solidifies at the tips of manganese sulfide inclusions
Calcium-deoxidized steels contain oxide flakes of calcium silicates (CaSO) that reduce the strength of the secondary shear zone and decrease tool–chip interface friction and wear

28. Machinability: Machinability of Ferrous Metals
Effects of Various Elements in Steels
Presence of aluminum and silicon is harmful, as it combine with oxygen to form aluminum oxide and silicates, which are hard and abrasive
Thus tool wear increases and machinability reduce
Stainless Steels
Austenitic (300 series) steels are difficult to machine
Ferritic stainless steels (also 300 series) have good machinability
Martensitic (400 series) steels are abrasive

29. Machinability: Machinability of Nonferrous Metals
Aluminum is very easy to machine
Beryllium requires machining in a controlled environment
Cobalt-based alloys require sharp, abrasion-resistant tool materials and low feeds and speeds
Copper can be difficult to machine because of builtup edge formation
Magnesium is very easy to machine, with good surface finish and prolonged tool life
Titanium and its alloys have very poor thermal conductivity
Tungsten is brittle, strong, and very abrasive

30. Cutting fluids Decreasing power requirement
Increasing heat dissipation
Neat oils+ extreme pressure additives
Water emulsions